Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is particularly well suited for application of characters onto substrates such as glass, metal, or plastic containers and packages.
Reviews of various aspects of ink jet printing can be found in these publications: Kuhn et al., Scientific American, April, 1979, 162-178; and Keeling, Phys. Technol., 12(5), 196-303 (1981). Various ink jet apparatuses are described in the following U.S. Pat. Nos. 3,060,429, 3,298,030, 3,373,437, 3,416,153, and 3,673,601.
In general, an ink jet ink composition must meet certain requirements to be useful in ink jet printing operations. These requirements relate to viscosity, resistivity, solubility, compatibility of components and wettability of the substrate. Further, the ink must be quick-drying and smear resistant, resist abrasion, and be capable of passing through the ink jet nozzle without clogging, and permit rapid cleanup of the machine components with minimum effort. In addition, the printed messages must have sufficient adhesion to the substrates.
Plastic substrates, particularly polyolefins, are often used in packaging. For example, oriented polypropylene (OPP) films are widely used as packaging materials such as the over-wraps for cigarette packages and cartons because of their ability to heat seal and provide good barrier properties in maintaining freshness and moisture content of the packaged goods. These and other low surface energy substrates present certain problems with respect to the quality and robustness of the printed message. The messages are sometimes rubbed off or scratched off during normal handling even with relatively moderate force if the adhesion of the message to the substrate is poor. In order to alleviate this problem, the plastic substrates are sometimes provided with a surface pretreatment such as oxidation, carona treatment or plasma treatment, or a coating of a primer. However, such pretreatments undesirably add to the cost of the package manufacturing process.
Further, OPP films often contain slip additives to provide adequate seal release or coefficient of friction. These additives are predominantly migratory fatty acid amides, which bloom to the film surface and provide the slip property. These additives are believed to be contributing factors in the degradation of ink adhesion over time as well as adhesion variation due to the age difference of the films. The presence of such additives makes the ink adhesion on low surface energy films even more challenging.
Some attempts have been made to provide ink compositions for improving the adhesion of the printed message to low surface energy plastic substrates, for example, by the use of certain acrylamides or chlorinated polypropylene resin; see, for example, U.S. Pat. Nos. 5,430,093 and 5,500,251. However, other approaches are desirable.
The foregoing indicates that there is a need for a jet ink composition that is suitable for printing on plastic surfaces, particularly low energy surfaces such as polyolefin surfaces, messages that have abrasion or scratch resistance in general, and in particular, messages that have fingernail scratch or finger rub resistance.
The invention provides such a composition. The advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.